To increase efficiency of amplifier circuits or power amplifiers within, for example, cellular handsets, envelope tracking may be used. In envelope tracking systems, the bias current of a power amplifier is dynamically altered so that the power amplifier is operated in a power-efficient state or operation mode for the present input signal and the presently required output power. The chosen bias current is, therefore, coupled with or dependent on the present envelope of the input signal.
For wide band signals of modern cellular systems, as for example long-term evolution (LTE) 20 MHz, the envelope of the input signal to the power amplifier may have a high bandwidth. Most of the signal energy of the amplitude modulated (AM) signal is contained within +/− the bandwidth (BW) of the transmit (TX) signal, i.e. within the bandwidth of the used channel. However, the high frequency components of the AM signal are still relevant to achieve an acceptable transmitter noise performance outside of the bandwidth of the transmitter channel, in order to comply with adjacent channel leakage ratio (ACLR) requirements of the system, to avoid spurious emissions or to not transmit excessive noise at the duplex distance (at the frequency of the received band). In order to comply with those requirements, the envelope tracking system is required to have a high bandwidth to be able to follow also fast envelope changes due to the high frequency components of the AM signal, resulting in power consuming envelope tracking systems, which counterbalances the benefits of the envelope tracking of the power amplifier and reduces the efficiency of the whole system. For present wide band systems, sufficient bandwidth of the envelope tracking system may be principally unfeasible, resulting in potentially unacceptable noise contributions to the amplified radio frequency signal. Hence, there is a requirement to increase a performance of envelope tracking systems.